ZFS Scrubs, Single Drives

[Gareth Hill]

Evening all,

I read Cyberjocks guide to Freenas and had a question regarding ZFS Scrubs, I have all of my Hard drives in single Storage arrays - Basically just hard drives, I do not need redundancy due to complete replication on a second system (also single drives) each drive is matched on both system and contain specific data one for each volume.

I have some questions regarding the ZFS storage.

Firstly with only 1 Drive "pools" do you get the benefits of ZFS data checking or does that require two or more drives.

Secondly is there any point in Scrubbing the drives, In the guide it mentions that you can scrub should you have the replication capacity for it, does this mean with single drives that it is not possible to scrub? Would there be an advantage to Scrubbing single drive "pools"?

If I haven't made myself clear I apologise and please feel free to ask any questions that would help.

 

[cyberjock]

Thanks for reading my guide!

You will still get the benefit of identifying errors in your data. Unfortunately you get none of the self-correction since there is no redundancy with single-disk pools. You could chose to do a "copies=2" zfs parameter, but you'd lose 50% of your disk space and if a disk starts failing there's no guarantee your second copy of the file would be readable.

Scrubbing has some validity.. it will identify problems with data you might not otherwise access. But as I said above it won't fix them. You'll get an email when you start racking up errors and you'll certainly lose files if you setup emailing in FreeNAS though.

I'm confused on why you says you can scrub if you have the replication capacity for it. Scrubs don't have anything to do with Replication(or snapshots).

If I were in your shoes I'd probably do monthly scrubs and leave it at that. Scrubs are pretty hard on disks and since you're only there to find errors and not fix them the "need" is less significant. I'd *definitely* do SMART monitoring and SMART testing so that if a disk starts going bad you know before you lose everything(assuming it isn't a sudden catastrophic failure).

 

[joeschmuck]

To answer your question...
You can and should scrub any ZFS pool, single drive or multiple drive. A scrub checks the integrity of your data, something you would probably want. The only thing you are missing out on is the redundancy of your data across many drives, but you apparently understand the risks.